Cangming Ke

Impact of the n+ emitter layer on the structural and electrical properties of p-type polycrystalline silicon thin-film solar cells

The effect of the phosphine (PH3) flow rate on the doping profile, in particular the peak doping concentration of the n+ emitter layer, of solid phase crystallised polycrystalline silicon thin-film solar cells on glass is investigated by electrochemical capacitance-voltage profiling. The peak n+ layer doping is found to increase with increasing PH3 gas flow, resulting in a shift of the p-n junction location towards the centre of the diode. The impact of the PH3 flow rate on the crystal quality of the poly-Si films is analysed using ultraviolet (UV) reflectance and UV/visible Raman spectroscopy. The impact of the PH3 flow rate on the efficiency of poly-Si thin-film solar cells is investigated using electrical measurements. An improvement in the efficiency by 46% and a pseudo energy conversion efficiency of 5% was obtained through precise control of the flow rate at an intermediate n+ emitter layer doping concentration of 1.0 × 1019 cm−3. The best fabricated poly-Si thin-film solar cell is also found to hav...

Investigation of Interdigitated Metallization Patterns for Polycrystalline Silicon Thin-Film Solar Cells on Glass

In this study, we evaluate the impact of interdigitated metallization patterns on the 1-sun performance of poly-Si thin-film solar cells on glass. We implement a model of the solar cell with an interdigitated metallization pattern in the simulation software Silvaco Atlas. Simulation and experimental results show consistently that the dominant factor for the performance of the metallization pattern is its impact on the current generation capability of the solar cell. For this reason, we study the effect of a variation of the emitter finger pitch, rear contact size, and carrier lifetime on current generation. For a pitch between 500 and 600 μm, the solar cell efficiency varies by less than 0.2% absolute. We also find that the optimum emitter finger pitch does not depend strongly on the charge carrier lifetime in the absorber layer.

Efficiency Potential of Rear Heterojunction Stripe Contacts Applied in Hybrid Silicon Wafer Solar Cells

A 2-D numerical analysis evaluating the silicon solar cell efficiency potential of rear-side heterojunction stripe contacts embedded in an AlOx/SiNx passivation stack is presented. Simulated implied efficiencies of correspondingly passivated wafers and device efficiencies of corresponding hybrid solar cells are stated and compared with more conventional solar cell device architectures (i.e., n-PERT and full-area hybrid heterojunction cells). One-dimensional (full-area) and 2-D (stripe contact) lifetime samples are investigated as experimental input for simulation calibration. The influence of the rear-contact area fraction and pitch on both surface recombination and series resistance are investigated. The solar cell efficiency potential (i.e., the implied efficiency) of rear heterojunction stripe contacts is calculated using measured and simulated effective carrier lifetime and series resistance data. For a given (measured) injection-dependent lifetime of the heterojunction and dielectric passivation layers used, the optimum contact geometry (contact area fraction and pitch) is predicted. Assuming either a 1-ms effective lifetime for an ideal a-Si:H(i)/μc-Si:H(p) repassivated lifetime sample (i.e., assuming no laser-induced Si crystal damage due to the contact opening process) or an experimentally observed 300-μs lifetime for an a-Si:H(i)/μc-Si:H(p) repassivated lifetime sample, the calculated 1-sun efficiency potential of rear heterojunction stripe contacts (with optimized rear contact geometry) is 25.8% and 22.9%, respectively. The simulated efficiencies of corresponding hybrid heterojunction cells (having a conventional diffused front-contact system) are 22.7% and 21.4%, respectively. This is significantly higher than the calculated efficiency potential of more conventional solar cell architectures, such as n-PERT cells (20.6%) and full-area hybrid heterojunction solar cells (20.7%).

Surface passivation investigation on ultra-thin atomic layer deposited aluminum oxide layers for their potential application to form tunnel layer passivated contacts

The surface passivation performance of atomic layer deposited ultra-thin aluminium oxide layers with different thickness in the tunnel layer regime, i.e., ranging from one atomic cycle (~0.13 nm) to 11 atomic cycles (~1.5 nm) on n-type silicon wafers is studied. The effect of thickness and thermal activation on passivation performance is investigated with corona-voltage metrology to measure the interface defect density D it(E) and the total interface charge Q tot. Furthermore, the bonding configuration variation of the AlO x films under various post-deposition thermal activation conditions is analyzed by Fourier transform infrared spectroscopy. Additionally, poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) is used as capping layer on ultra-thin AlO x tunneling layers to further reduce the surface recombination current density to values as low as 42 fA/cm2. This work is a useful reference for using ultra-thin ALD AlO x layers as tunnel layers in order to form hole selective passivated contacts for silicon solar cells.

On the use of a charged tunnel layer as a hole collector to improve the efficiency of amorphous silicon thin-film solar cells

A new concept, using a negatively charged tunnel layer as a hole collector, is proposed and theoretically investigated for application in amorphous silicon thin-film solar cells. The concept features a glass/transparent conductive oxide/ultra-thin negatively charged tunnel layer/intrinsic a-Si:H/n-doped a-Si:H/metal structure. The key feature of this so called t+-i-n structure is the introduction of a negatively charged tunnel layer (attracting holes from the intrinsic absorber layer), which substitutes the highly recombination active p-doped a-Si:H layer in a conventional p-i-n configuration. Atomic layer deposited aluminum oxide (ALD AlOx) is suggested as a potential candidate for such a tunnel layer. Using typical ALD AlOx parameters, a 27% relative efficiency increase (i.e., from 9.7% to 12.3%) is predicted theoretically for a single-junction a-Si:H solar cell on a textured superstrate. This prediction is based on parameters that reproduce the experimentally obtained external quantum efficiency and cu...

Theoretical exploration of Cd-free CIGS solar cells with a charged tunneling electron collector layer

Beyond the common cadmium-free copper indium gallium (di) selenide (CIGS) solar cell made with a physical buffer layer of zinc sulfide (ZnS) or indium sulfide (In2S3), a newly devised Cd-free cell using a charged tunneling layer is explored theoretically in this work. In principle, applying an ultra-thin tunnel layer (for example by atomic layer deposition) with a high positive fixed interface charge density, the conventional buffer layer can be substituted. Assuming a fixed interface charge density of 1013 cm-2 of the tunneling layer, the simulated CIGS efficiency is over 15%, being close to the reference CIGS solar cell with CdS buffer layer. The benefits and limitations of the Cd-free tunneling layer CIGS solar cells are discussed. Potential processes suitable for realizing the tunneling layer are also discussed.

Laser Chemical Processing of n - Type Emitters for Solid-Phase Crystallized Polysilicon Thin-Film Solar Cells

We report on the application of laser chemical processing (LCP) to fabricate n-type emitters for polysilicon thin-film solar cells on glass. Sheet resistance values of 2-5 kΩ/□ with a peak phosphorus doping concentration in the range 8 × 10¹⁸ -1 × 10¹⁹ cm^-3 at a shallow doping depth of less than 350 nm are achieved. After dopant activation and a hydrogenation process, the best cell has an average V_oc of (446 ± 7) mV and a pseudofill factor (pFF) of (68.3 ± 0.9)%. This paper demonstrates that LCP can be successfully applied to fabricate an active layer for polysilicon thin-film solar cells on glass. Further improvement in the V_oc and the pFF may be possible by optimizing the post-LCP annealing and hydrogenation process, as well as using a poly-Si film of superior material quality.